displaying images in in PDF's with fop

[api]

I'm using XSL stylesheets and XML documents to generate PDF files with
xsltproc, then fop. The problem is, I can't get images to show up in
the PDF (the text showing up in the fo file, though). I've tried the
following:

<fo:external-graphic src="url(/path/to/image.jpg)" />

<img src="http://path/to/image.jpg" content-height=".2in"
content-width=".5in" overflow="visible"/>

<fo:external-graphic src="file:///http://www.path.to.image.jpg"
content-height="70%" content-width="70%" overflow="visible"/>

<fo:external-graphic width="50pt" height="50pt" overflow="hidden"
src="image.jpg"/>

I also tried a number of other variations of <img src>,
<fo:external-graphic>, and even <mediaobject>. I tried absolute and
relative paths. I've seen a lot of posts from people about getting
images to display in their PDFs, but the solution was generally one of
the ones I listed above. Am I missing something?

Thanks.